Some caveman sodomizes an ET with a mammoth tusk millennia ago and now we're barred from the galactic federation. Hope it was worth it, you unga bunga asshole.
seriously though I've always wondered why people never talk about the possibility of having been visited in prehistoric times, or even before humanity, when they talk about why we haven't seen aliens. maybe I'm missing some of the conversation but speaking for myself i haven't seen this possibility discussed much when people talk about Fermi's paradox for example